Compound semiconductor epitaxial substrate and method for manufacturing the same

ABSTRACT

In a compound semiconductor epitaxial substrate used for a strain channel high electron mobility field effect transistor which comprises an InGaAs layer as a channel layer  9  and AlGaAs layers containing n-type impurities as electron supplying layers  6  and  12 , the channel layer  9  has an electron mobility at room temperature of 8300 cm 2 /V·s or more by adjusting an In composition of the InGaAs layer composing the channel layer  9  to 0.25 or more and optimizing the In composition and the thickness of the channel layer  9 . GaAs layers  8  and  10  having a thickness of 4 nm or more each may be laminated respectively in contact with a top surface and a bottom surface of the channel layer  9.

TECHNICAL FIELD

The present invention relates to a compound semiconductor epitaxialsubstrate used for a pseudomorphic high electron mobility field effecttransistor composed of a III-V compound semiconductor, and a method formanufacturing the same.

BACKGROUND ART

An electronic device utilizing a GaAs III-V compound semiconductor hasactively applied to an ultra-high speed transistor, taking advantage offeatures such as an ability to operate at an ultra-high speed and at ahigher frequency because of its high electron mobility, and has recentlybeen used practically as various essential components of high-frequencycommunication instruments such as a cell phone because of its advantageof low power consumption.

As the above described ultra-high speed transistor, a high electronmobility field effect transistor (referred to as a HEMT, hereinafter)has been well known. The HEMT is also referred to as a high electronmobility transistor, a modulation doped field effect transistor(MODFET), or a hetero-junction field effect transistor (HJFET).

The HEMT is principally characterized by adopting a selective dopedhetero structure which is composed of an electron supplying layer forsupplying electrons (a doped layer) and a channel layer through whichelectrons run, these layers being made of different materials. In thishetero structure, electrons supplied from n-type impurities within theelectron supplying layer are pooled in a potential well formed at achannel side of a hetero-junction interface due to a difference ofelectron affinity between materials constituting the hetero junction toform a two-dimensional electron gas. Since n-type impurities supplyingelectrons are present in the electron supplying layer while electronsare present in a high purity channel as described above so that theionized impurities and the electrons are spatially separated from eachother, the two-dimensional electron gas within the channel is hardlyscattered by the ionized impurities, and consequently a high electronmobility is realized.

The HEMT has usually been fabricated by using an epitaxial substrate inwhich thin film crystal layers respectively having predeterminedelectronic characteristics are laminated and grown on a GaAs singlecrystal substrate so as to have a predetermined structure. It isimportant for the HEMT to have a high electron mobility in the channel.Therefore, since it has been required to precisely control the thin filmcrystal layer forming the HEMT structure on the order of monoatomiclayer level, a molecular beam epitaxy (referred to as a MBE method,hereinafter) or a metalorganic chemical vapor deposition (referred to asa MOCVD method, hereinafter) has conventionally been used as a methodfor manufacturing an epitaxial substrate having a HEMT structure.

It has been said that the MBE method, which is one of the vacuumevaporation methods, is excellent in the layer thickness and interfacecontrollability, while this MBE method is inferior in themass-productivity. On the contrary, the MOCVD method uses anorganometallic compound or a hydride of atomic species constituting anepitaxial layer as a source material and then pyrolyzes the sourcematerial on the single crystal substrate to grow a crystal thereof, sothat this method is applicable to a wide range of substances used as thesource materials and has a feature of extremely extensively andprecisely controlling the composition and thickness of the epitaxialcrystal, and consequently this method is suitable for the purpose ofprocessing a large amount of substrates with the favorablereproducibility.

In addition, recent and rapid technical innovation of the MOCVD methodhas allowed for not only controlling of the impurity amounts but alsorealizing a steep hetero interface or favorable in-plane uniformity thathas never been considered as possible by this method. In fact, anepitaxial substrate fabricated by the MOCVD method is in no way inferiorto that fabricated by the MBE method in terms of a characteristic suchas an electron mobility of the HEMT, and has commercially and widelybeen used.

Thus, since the HEMT is an ultra-high speed transistor utilizing atwo-dimensional electron gas having a high electron mobility, anelectron mobility of the channel layer which is as high as possible isfavorable for obtaining a high-performance HEMT. Therefore, InGaAs hasrecently been used as a material for the channel layer instead of usingGaAs, because InGaAs is excellent in its electron transportingcharacteristic as well as being capable of significantly changing itsenergy gap in accordance with the In composition and further capable ofeffectively confining the two-dimensional electrons. In addition, AlGaAsor GaAs is known as a material to be combined with InGaAs.

InGaAs has a property such that the mobility becomes higher with theincrease of the In composition. Thus the transistor can besophisticated, however, a lattice constant of InGaAs also becomes largerwith the increase of the In composition, which leads to lattice misfitwith the electron supplying layer or the substrate material. For thisreason, a method in which crystal growth is performed in a pseudomorphiccondition has been used. This method utilizes a property in which a highquality crystal layer can be fabricated without inducing latticedisorder such as dislocation although the lattice strains andelastically deformed, provided that the grown layer thickness is below acertain layer thickness referred to as a critical thickness even if thecrystal growth involves the lattice misfit between materials havingdifferent lattice constants. The HEMT, in which an InGaAs strain layeris used as a channel layer, is referred to as a pseudomorphic highelectron mobility field effect transistor (hereinafter, referred to as apHEMT).

It has been known that a critical thickness of the InGaAs layer is givenas a function of the In composition and the layer thickness. As for anInGaAs layer with respect to a GaAs layer for example, the criticalthickness is expressed by a theoretical equation disclosed in J. CrystalGrowth, 27 (1974) p. 118 and in J. Crystal Growth, 32 (1974) p. 265, andthis theoretical equation has been found to be experimentally correct asa whole. In addition, JP-A-6-21106 discloses an epitaxial substratewhich allows for efficient manufacture of a pHEMT having a highmobility, the epitaxial substrate having an In composition within acertain range which is further limited by using a certain relationalexpression based on a certain relation between an In composition and alayer thickness defined by this theoretical equation. Indeed, an InGaAslayer having an In composition of 0.20 and a layer thickness of about 13nm has been practically used as an InGaAs strain channel layer which canbe epitaxially grown without inducing a reduction in its crystallinity.

Further, it is effective to additionally reduce the scattering oftwo-dimensional electrons caused by ionized impurities in order toimprove the mobility. Thus, a spacer layer which has the same materialand composition as an electron supplying layer but to which anyimpurities are not added may be inserted between the electron supplyinglayer and the channel layer. For example, Japanese Patent No. 2708863discloses a structure for improving a two-dimensional electron gasconcentration and an electron mobility, in which a spacer layerconsisting of an AlGaAs layer and a GaAs layer is inserted between anInGaAs strain layer used as a channel layer and an n-AlGaAs electronsupplying layer of a pHEMT structure in order to optimize the growthconditions.

A pHEMT which uses a strain InGaAs layer as a channel layer as describedabove can confine a large number of two-dimensional electrons under theeffective influence of quantum effect by making an In composition of thechannel layer larger and making a band gap between the channel layer andthe electron supplying layer or spacer layer wider, so that the pHEMThas an advantage that the improvement of electron mobility can becompatible with the high mobility.

It is well known that the electron mobility is an important parameterfor improving various characteristics such as an on-resistance, amaximum current value, or a transconductance each of which is animportant performance indicator of a field effect transistor. Therefore,further improvement of the electron mobility can achieve a reduction ofbuild-up resistance (on-resistance), which leads to a reduction of powerconsumption. In addition, since a calorific value can be decreased bylowering the power consumption, it is possible not only to realizehigher integration of a device but also to reduce the chip size, so thatthe number of chips manufactured from one epitaxial substrate can beincreased and a degree of freedom of modular design can also beincreased. From this viewpoint, it is desired to further improve theelectron mobility in the case of a pHEMT which is used for variousportable instruments such as a cell phone.

However, the electron mobility in the epitaxial substrate having thepHEMT structure has not yet reached a satisfactory level, in view ofpossibility that the characteristics of the transistor can be furtherimproved by increasing the two-dimensional electron gas concentrationand the electron mobility simultaneously. For example, as described in“Compound Semiconductor, Materials Head into Mass Production Stages(2)”—Compound Semiconductor Materials for Electronic Devices—, YoheiOtoki and in Semiconductor Industry News Forum “All about CompoundSemiconductors 2002—Movement of Optical High Frequency Devices towardReemerge”, Jun. 5, 2002, Myoujin Kaikan, Ochanomizu, Tokyo, a maximumvalue of the electron mobility in the channel layer at room temperature(300K) which had been reported with respect to the pHEMT structureepitaxial substrate was about 8170 cm²/V·s at a two-dimensional electronconcentration of 2.06×10¹²/cm², and about 7970 cm²/V·s at atwo-dimensional electron concentration of 2.77×10¹²/cm².

The electron mobility in the two-dimensional electron gas at roomtemperature (300K) has been considered to be determined by thescattering due to a crystal lattice and the effective mass of the GaAselectrons. Thus, if a strain InGaAs layer is used as a channel layer towhich In is added, it is expected that the effective mass of electronsdecreases and the electron mobility increases, but on the contrary,there is concern that an increase in alloy scattering due to In and Gamay result in a reduction in the electron mobility. Further, theeffective mass of electrons has been considered to develop anisotropyalong a vertical direction and a horizontal direction with respect to atwo-dimensional electron gas plane, and there has not yet been reportedan effective mass along the horizontal direction which is practicallyimportant. Thus, under the present circumstances, any measures have notbeen taken which ensure a reduction of the effective mass of electronsand the improvement of electron mobility.

On the other hand, it has already been reported that InGaAs which haslattice-matched with an InP single crystal substrate to be used has anelectron mobility of 10000 cm²/V·s at room temperature. In addition, theso-called metamorphic technology has recently been developed for forminga buffer layer whose lattice constant is near a lattice constant of InPby changing a lattice constant of the buffer layer to a lattice constantof InP in stages, provided that an InAlAs is used as a buffer layer onthe GaAs single crystal substrate and an In mixed crystal ratio ischanged in stages. An electron mobility exceeding 9000 cm²/V·s has beenreported by forming a modulation doped structure having an InGaAs layeras a channel on the buffer layer formed by using this technology. Thatis, InGaAs without strain channel allows for a higher electron mobilitywhich exceeds GaAs and is consistent with a low electron effective massthereof.

When a high-performance transistor having a high electron mobility suchas exceeding 8200 cm²/V·s is achieved as described above, it isnecessary to use an InP single crystal substrate or to laminate aspecial buffer layer on a GaAs single crystal substrate in accordancewith the above described metamorphic technology. However, if the InPsingle crystal substrate is used, it is necessary to use an epitaxialsubstrate having a modulation doped structure comprising as a channellayer an InGaAs layer being lattice-matched with InP and as an electronsupplying layer an InAlAs layer being lattice-matched with this channellayer. For this reason, source material cost becomes extremely high whenan InP single crystal substrate is used. On the other hand, even themetamorphic technology to be used, in which a thick buffer layer isformed, has a problem that the production cost becomes larger and also anew material processing technology which is different from that in thecase of conventional GaAs material is required, and further has aproblem that a low reliability is provided due to a high crystal defectdensity in the buffer layer.

Thus, in the pHEMT structure epitaxial substrate which uses an n-AlGaAslayer as an electron supplying layer and a strain InGaAs layer as achannel layer, a further improved epitaxial substrate is stronglydesired which has a higher two-dimensional electron gas concentrationtogether with a higher electron mobility compared with the currentlyreported values.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an epitaxial substrateused for a pHEMT having a high electron mobility essential to achievethe above described various necessary characteristics and a method formanufacturing the same.

The present inventors have devoted ourselves to solve the abovedescribed problems, and have consequently found that an epitaxialsubstrate used for a pHEMT structure in which an In composition and athickness of an InGaAs channel layer are optimized and further athickness of a spacer layer comprising an AlGaAs layer or a GaAs layerprovided between the InGaAs channel layer and an n-AlGaAs electronsupplying layer is optimized has a higher electron mobility togetherwith a higher two-dimensional electron gas concentration which havenever been reported before, and have now achieved the present inventionbased on such findings.

According to a first aspect of the present invention, there is provideda compound semiconductor epitaxial substrate used for a strain channelhigh electron mobility field effect transistor, comprising an InGaAslayer as a strain channel layer and an AlGaAs layer containing n-typeimpurities as an electron supplying layer, wherein the InGaAs layer hasan electron mobility at room temperature of 8300 cm²/V·s or more. Atwo-dimensional electron gas concentration in the above described strainchannel layer at room temperature may be 2.20×10¹²/cm² or more.

According to a second aspect of the present invention, there is providedthe compound semiconductor epitaxial substrate as described in the abovefirst aspect, wherein the InGaAs layer comprising the above describedstrain channel layer was an In composition of 0.25 or more.

According to a third aspect of the present invention, there is providedthe compound semiconductor epitaxial substrate as described in the abovesecond aspect, wherein GaAs layers having a thickness of 4 nm or moreeach are laminated on the above described strain channel layerrespectively in contact with a top surface and a bottom surface of theabove described strain channel layer.

According to a fourth aspect of the present invention, there is provideda method for manufacturing the compound semiconductor epitaxialsubstrate as described in the above first, second, or third aspect,comprising epitaxially growing respective compound semiconductor layersemploying a metalorganic chemical vapor deposition (MOCVD) method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing a layer structure of an epitaxial substrateaccording to Example 1 of the present invention;

FIG. 2 is a graph of experimental results for showing an effect of thepresent invention, and showing a relation between an In composition ofthe channel layer and an electron mobility in the channel layer at roomtemperature;

FIG. 3 is a drawing showing a layer structure of an epitaxial substrateaccording to Example 2 of the present invention; and

FIG. 4 is a drawing showing a layer structure of an epitaxial substrateaccording to Comparative Example 1 of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

One example of the present invention will now be described in detailwith reference to the drawings.

FIG. 1 is a drawing for describing a cross-sectional structure of afirst example of a HEMT structure epitaxial substrate according to thepresent invention. In FIG. 1, a reference numeral 1 denotes a GaAssingle crystal substrate which is a single crystal substrate and each ofreference numerals 2 to 5 denotes a buffer layer laminated on the GaAssingle crystal substrate 1. The buffer layers 2 to 5 herein arelaminated respectively as an i-GaAs layer having a thickness of 200 nm,an i-Al_(0.25)Ga_(0.75)As layer having a thickness of 250 nm, an i-GaAslayer having a thickness of 250 nm, and an i-Al_(0.25)Ga_(0.75)As layerhaving a thickness of 200 nm.

Reference numeral 6 denotes a back side electron supplying layer formedas an n-Al_(0.24)Ga_(0.76)As layer having a thickness of 4 nm and dopedwith n-type impurities at a concentration of 3×10¹⁸/cm³. Back sidespacer layers 7 and 8 are formed in this order on the back side electronsupplying layer 6. The back side spacer layer 7 herein is ani-Al_(0.24)Ga_(0.76)As layer having a thickness of 3 nm and the backside spacer layer 8 is an i-GaAs layer having a thickness of 6 nm.Reference numeral 9 denotes a channel layer in which two-dimensionalelectron gases are formed in order to flow two-dimensional electronstherethrough, and is an i-In_(0.30)Ga_(0.70)As layer having a thicknessof 7.5 nm.

Each of reference numerals 10 and 11 denotes a front side spacer layer,the front side spacer layer 10 being an i-GaAs layer having a thicknessof 6 nm and the front side spacer layer 11 being ani-Al_(0.24)Ga_(0.76)As layer having a thickness of 3 nm.

Reference numeral 12 denotes a front side electron supplying layerformed as an n-Al_(0.24)Ga_(0.76)As layer having a thickness of 10 nmand doped with n-type impurities at a concentration of 3×10¹⁸/cm³.Reference numerals 13 and 14 denote undoped layers which are formed asan i-Al_(0.22)Ga_(0.78)As layer having a thickness of 3 nm and an i-GaAslayer having a thickness of 20 nm respectively.

Since the epitaxial substrate shown in FIG. 1 is laminated and grown asdescribed above, electrons are supplied from the back side electronsupplying layer 6 to the channel layer 9 through the back side spacerlayers 7 and 8, as well as supplying electrons from the front sideelectron supplying layer 12 to the channel layer 9 through the frontside spacer layers 11 and 10. Consequently, a high concentration of atwo-dimensional electron gas is formed in the channel layer 9.

Since an In composition of the channel layer 9 herein is adjusted to 0.3which is larger than 0.25 and further a thickness of the channel layer 9is adjusted to 7.5 nm in order to optimize the In composition and thethickness of the channel layer 9, the two-dimensional electron gasconcentration within the channel layer 9 can be increased in addition tobeing capable of dramatically improving the electron mobility of thetwo-dimensional electron gas compared with the prior art. Consequently,even when a two-dimensional electron gas concentration in the channellayer 9 at room temperature (300K) is adjusted to 2.20×10¹²/cm² or more,an electron mobility at this concentration can be at least 8300 cm²/V·sor more.

Although the embodiment shown in FIG. 1 has been described with respectto a structure in which an In composition of the channel layer 9 made ofan InGaAs strain channel layer is 0.3 by way of example, a relation asshown in FIG. 2 was obtained as a result of investigating a relationbetween the In composition and the electron mobility of the channellayer while adjusting the In composition to various levels so that thechannel layer thickness became optimum. In the measurement shown in FIG.2, a thickness of the channel layer 9 was adjusted to about 80% of acritical layer thickness which was determined from a theoreticalequation described in J. Crystal Growth, 27 (1974) p. 118 and J. CrystalGrowth, 32 (1974) p. 265 by using the In composition of the channellayer 9. When an In composition of the channel layer 9 was 0.25 or more,a thickness of the channel layer 9 was 10.5 nm or less. In addition, atwo-dimensional electron concentration at each datum point as shown inFIG. 2 was adjusted to (2.2 to 2.4)×10¹²/cm².

As described above, in a pHEMT structure in which a channel layer madeof a non-doped InGaAs layer and an electron supplying layer made of anAlGaAs layer containing n-type impurities are epitaxially grown, it hasbeen confirmed that a two-dimensional electron gas concentration can beincreased in addition to being capable of obtaining an electron mobilityof the two-dimensional electron gas within the channel layer of 8300cm²/V·s or more at room temperature (300K) by adjusting an Incomposition of the channel layer to 0.25 or more and optimizing athickness of this layer. In this case, it has been confirmed that eachof GaAs layers laminated respectively in contact with a top surface anda bottom surface of the channel layer may have a thickness of 4 nm ormore.

Then, a method for fabricating the epitaxial substrate having a layerstructure as shown in FIG. 1 will be described. First, a GaAs singlecrystal substrate 1 is prepared. The GaAs single crystal substrate 1 isa high-resistive semi-insulating GaAs single crystal substrate, and itis favorable to use a GaAs substrate manufactured by a LEC (LiquidEncapsulated Czochralski) method, a VB (Vertical Bridgeman) method, aVGF (Vertical Gradient Freezing) method or the like. In any of thesemanufacturing methods, it should be provided a substrate having an angleof inclination from about 0.05° to 10° with respect to onecrystallographic plane direction.

A surface of the GaAs single crystal substrate 1 prepared as describedabove is degreased and washed, etched, rinsed, and dry-processed, andsubsequently this substrate is placed on a heating table of a crystalgrowth reactor. An inside of the reactor is substantially substitutedwith high purity hydrogen before starting a heating operation. Once amoderate and stable temperature is reached within the reactor, arsenicsource materials are introduced thereto. Subsequently, gallium sourcematerials are introduced thereto when a GaAs layer is grown. Further,gallium source materials and aluminum source materials in addition toarsenic source materials are introduced thereto when an AlGaAs layer isgrown. When an InGaAs layer is grown, gallium source materials andindium source materials are introduced thereto in addition to theintroduction of arsenic source materials. The desired laminatedstructure is being grown by controlling a supplying amount and asupplying time of each source material. Finally, the supply ofrespective source materials are terminated to stop the crystal growth,and after the cooling operation, the epitaxial substrate formed by thelamination as shown in FIG. 1 is removed from the reactor to completethe crystal growth. A substrate temperature at a time of growingcrystals is usually from about 500° C. to 800° C.

An epitaxial substrate having a layer structure as shown in FIG. 1 canbe fabricated by employing a MOCVD method. The advantage of employingthe MOCVD method is that the metalorganic compounds or hydrides ofatomic species constituting the epitaxial layer can be used as thesource materials.

Practically, arsenic trihydride (arsine) is frequently used as anarsenic source material during the epitaxial growth, however, it is alsopossible to use alkyl arsine which is obtained by substituting hydrogenof arsine with an alkyl group having 1 to 4 carbon atoms. As gallium,aluminum, and indium source materials, it is generally used atrialkylate or a trihydride which is obtained by bonding an alkyl grouphaving 1 to 3 carbon atoms or hydrogen to each metal atom.

As an n-type dopant, it is possible to use a hydride or an alkylatehaving an alkyl group whose carbon number is 1 to 3 of silicon,germanium, tin, sulfur, selenium or the like.

The present invention will now be described in detail based on thecomparison of Examples with Comparative Example, however, the presentinvention is not limited to these examples.

Example 1

An epitaxial substrate having a layer structure shown in FIG. 1 wasfabricated as described below by the use of a low pressure barrel-typedMOCVD reactor.

A semi-insulating GaAs single crystal substrate manufactured by the VGFmethod was prepared as a GaAs single crystal substrate 1, and respectivecompound semiconductor layers were epitaxially grown on the GaAs singlecrystal substrate 1. Trimethyl gallium (TMG), trimethyl aluminum (TMA),and trimethyl indium (TMI) were used as source materials of thethird-group elements, while arsine (AsH₃) was used as a source materialof the fifth-group element. Silicon (Si) was used as an n-type dopant.High-purity hydrogen was used as a carrier gas for the source materials,and the epitaxial growth was performed under the conditions that apressure within the reactor was 0.1 atm, a growth temperature was 650°C., and a growth rate was 3 to 1 μM/hr.

The channel layer 9 through which electrons run was epitaxially grown inorder to obtain a strain InGaAs layer having an In composition of 0.30and a thickness of 7.5 nm.

Each of i-GaAs layers as spacer layers 8 and 10 was epitaxially grown toa thickness of 6.0 nm on a top surface and a bottom surface of theInGaAs layer used as the channel layer 9, respectively.

According to the epitaxial substrate obtained as described above, theresult of performing the hall measurement in accordance with a Van derPauw method showed better measurement values which had never beenreported before, that is, a two-dimensional electron gas concentrationat room temperature (300K) was 2.28×10¹²/cm², an electron mobility atroom temperature (300K) was 8990 cm²/V·s, a two-dimensional electron gasconcentration at 77K was 2.59×10¹²/cm², and an electron mobility at 77Kwas 35600 cm²/V·s. In addition, as a result of performing a CVmeasurement by using an Al schottky electrode with respect to thisstructure, a pinch-off voltage at a residual carrier concentration of1×10¹⁵/cm³ was −1.93 V.

Example 2

An epitaxial substrate having a layer structure shown in FIG. 3 wasfabricated in accordance with the MOCVD method as in the case ofExample 1. In the epitaxial substrate shown in FIG. 3, a referencenumeral 21 denotes a semi-insulating GaAs single crystal substrate,reference numerals 22 to 25 denote buffer layers, a reference numeral 26denotes a back side electron supplying layer, reference numerals 27 and28 denote back side spacer layers, a reference numeral 29 denotes achannel layer, reference numerals 30 and 31 denote front side spacerlayers, a reference numeral 32 denotes a front side electron supplyinglayer, a reference numeral 33 denotes an undoped AlGaAs layer, and areference numeral 34 denotes an undoped GaAs layer. Composition andthickness of each layer are as shown in FIG. 3.

As can be seen from comparing FIG. 3 with FIG. 1, Example 2 is differentfrom Example 1 in that a strain InGaAs layer having an In composition of0.35 and a thickness of 5.5 nm is epitaxially grown. Formation of otherrespective layers are the same as in the case of Example 1.

According to the epitaxial substrate obtained as described above, theresult of performing the hall measurement in accordance with a Van derPauw method showed favorable measurement values, that is, atwo-dimensional electron gas concentration at room temperature (300K)was 2.22×10¹²/cm², an electron mobility at room temperature (300K) was8950 cm²/V·s, a two-dimensional electron gas concentration at 77K was2.22×10¹²/cm², and an electron mobility at 77K was 33000 cm²/V·s. Inaddition, as a result of performing a CV measurement by using an Alschottky electrode with respect to this structure, a pinch-off voltageat a residual carrier concentration of 1×10¹⁵/cm³ was −1.75 V.

Comparative Example

As for a pHEMT structure epitaxial substrate, an epitaxial substratehaving a structure shown in FIG. 4 was fabricated by employing the MOCVDmethod in the same way as above, except only for making modifications toan In composition and a thickness of the strain InGaAs layer used forthe channel layer and to a thickness of the i-GaAs layers laminated onthe top and bottom surfaces of this channel layer. In the epitaxialsubstrate shown in FIG. 4, a reference numeral 41 denotes asemi-insulating GaAs single crystal substrate, reference numerals 42 to45 denote buffer layers, a reference numeral 46 denotes a back sideelectron supplying layer, reference numerals 47 and 48 denote back sidespacer layers, a reference numeral 49 denotes a channel layer, referencenumerals 50 and 51 denote front side spacer layers, a reference numeral52 denotes a front side electron supplying layer, a reference numeral 53denotes an undoped AlGaAs layer, and a reference numeral 54 denotes anundoped GaAs layer. Composition and thickness of respective layers areas shown in FIG. 4.

As can be seen from comparing FIG. 4 with FIG. 3, Comparative Example isdifferent from a structure shown in FIG. 3 (Example 2) in that thechannel layer 49 has an In composition of 0.20 and a thickness of 13.5nm, and that a thickness of each of i-GaAs layers serving as a spacerlayer and provided on a top surface and a bottom surface of the channellayer 49 is 2.0 nm, respectively. The structure of Comparative Exampleas shown in FIG. 4 represents a pHEMT structure which has conventionallybeen well known.

As for the epitaxial substrate obtained as described above, the resultof performing the hall measurement in accordance with a Van der Pauwmethod showed that the measurement values were almost the same as valueswhich had been reported before, that is, a two-dimensional electron gasconcentration at room temperature (300K) was 2.55×10¹²/cm², an electronmobility at room temperature (300K) was 7200 cm²/V·s, a two-dimensionalelectron gas concentration at 77K was 2.78×10¹²/cm², and an electronmobility at 77K was 21900 cm²/V·s. In addition, as a result ofperforming a CV measurement by using an Al schottky electrode withrespect to this structure, a pinch-off voltage at a residual carrierconcentration of 1×10⁵/cm³ was −2.12 V.

Each of the layer structures of the epitaxial substrates from Examplesand Comparative Example is a test structure for evaluating the mobilityfollowing the Hall measurement and for evaluating the two-dimensionalelectron gas characteristics such as the threshold voltage measurementfollowing the CV measurement. In a layer structure of an actualepitaxial substrate used for fabricating a FET device, a thickness of anon-doped GaAs layer corresponding to the fourteenth layer in the layerstructure of the epitaxial substrate from Examples and ComparativeExample is increased, and in addition, contact layers are laminated forachieving ohmic contacts with a source electrode and a drain electrode.As the contact layer, it is usually used an n-GaAs layer which is dopedwith silicon at a concentration about 3×10¹⁸ to 5×10¹⁸/cm³ and laminatedto a thickness of about 100 nm. However, the effect of improving themobility according to the present invention is never compromised by aprocess for growing the contact layers and for fabricating the FETdevice. The effect of improving the mobility according to the presentinvention is significant not only in the test structures for evaluatingthe characteristics of epitaxial substrates from Examples ComparativeExample, but also in a structure of an epitaxial substrate used for theFET device.

High electron mobility which can be achieved for the first time in astrain InGaAs channel system on a GaAs substrate according to thepresent invention has a significant impact on the art, because such ahigh electron mobility has never been obtained except in a system inwhich an InGaAs layer is used as a channel layer and lattice-matchedwith an InP substrate, or alternatively in a system in which an InGaAslayer is used as a channel layer and lattice-matched with a buffer layerhaving almost the same lattice constant as that of InP formed by ametamorphic technology on a GaAs substrate. That is, the presentinvention has an commercially significant advantage of its applicabilityto the conventional device processing technology without making anymodifications, because the present invention requires neither an InPsubstrate which is expensive and hard to deal with nor a specialmetamorphic buffer technology and because an electron supplying layer orbuffer layer is basically the same as that of the conventional pHEMT.

In addition, since application of the HEMT is controlled by an electronvelocity which is closely correlated with the electron mobility, a pHEMTwhich uses a GaAs substrate of the present invention is to open up newpossibilities for realizing various high-speed devices operated withinan ultra high frequency band from several tens of GHz to severalhundreds of GHz, in which the realization of the pHEMT using a GaAssubstrate of the present invention has been said to be difficult unlessan InP substrate is used or a metamorphic technology is used. For thisreason, the present invention is to bring a great deal of advantages tothe art.

INDUSTRIAL APPLICABILITY

According to the present invention, there is provided a compoundsemiconductor epitaxial substrate having a pHEMT (strain channel highelectron mobility field effect transistor) structure with the favorablecharacteristics which have never been reported before and beingadvantageous to the fabrication of electronic devices, as describedabove.

1. A compound semiconductor epitaxial substrate for use in a strainchannel high electron mobility field effect transistor, comprising anInGaAs layer as a strain channel layer and an AlGaAs layer containingn-type impurities as an electron supplying layer, wherein said InGaAslayer has an electron mobility at room temperature of 8300 cm²/V·s ormore, wherein undoped GaAs layers having a thickness of 4 nm or moreeach are laminated respectively in contact with a top surface and abottom surface of said strain channel layer; wherein at least one ofsaid undoped GaAs layers is further in contact with an undoped AlGaAslayer; and wherein said AlGaAs layer containing n-type impurities is incontact with said undoped AlGaAs layer.
 2. The compound semiconductorepitaxial substrate according to claim 1, wherein the InGaAs layerconstituting said strain channel layer has an In composition of 0.25 ormore.
 3. The compound semiconductor epitaxial substrate according toclaim 1, wherein the InGaAs layer constituting said strain channel layerhas an In composition of 0.35 or more.